1. Field of the Invention
This invention is related to an inverter apparatus in which a commercial AC power supply is converted into a DC voltage, and this DC voltage is switched by turning ON/OFF switching means, so that high frequency power is supplied to a load.
2. Description of the Related Art
FIG. 11 is a circuit diagram of a conventional inverter apparatus conceivable from, for instance, Japanese Patent Disclosure No. Hei-2-211065 and Japanese Patent Disclosure No. Hei-4-193064. FIGS. 12A and 12B represent operation waveform diagrams of this circuit.
In FIG. 11, after a DC voltage obtained by rectifying a commercial power supply 1 by way of a rectifying circuit 2 is smoothed by a smoothing capacitor 3, this DC voltage is applied to series-connected transistors 4 and 5. The transistors 4 and 5 are controlled by a control circuit 6 in such a manner that these transistors are alternately turned ON/OFF in a high speed, so that high frequency power is supplied from a connection point between the transistors 4 and 5 to a load circuit 7.
In this example, the load circuit 7 is arranged by, for instance, a coupling capacitor 8, a current limiting coil 9, a discharge lamp 10, and a capacitor 11 connected in parallel to the discharge lamp 10. Filaments 101 and 102 are built in both ends of the discharge lamp 10, and the high frequency power causes a current to flow through the discharge lamp 10 and also to flow via a capacitor 11 through the filaments 101 and 102, so that these filaments 101 and 102 are heated.
In FIGS. 12A and 12B, FIG. 12A represents an input voltage waveform from the commercial power supply 1, and FIG. 12B indicates an input current waveform from the commercial power supply 1.
Operations will now be made.
In such a conventional inverter apparatus, when the transistor 4 is brought into the ON state and the transistor 5 is brought into the OFF state, the current will flow from the smoothing capacitor 3 via the transistor 4 to the load circuit 7 along one direction, and this current charges the coupling capacitor 8 provided in the load circuit 7.
Also, when the transistor 4 is brought into the OFF state and the transistor 5 is brought into the ON state, the charges of the coupling capacitor 8 are discharged via the transistor 5, and then the current will flow into the load circuit 7 along a direction opposite to the above-described direction.
As a result, the transistors 4 and 5 are alternately turned ON/OFF at a high speed by the control circuit 6, so that the high frequency power is supplied to the load, namely the discharge lamp 10.
However, as shown in FIGS. 12A and 12B, in the conventional circuit, when the power supply voltage of the commercial power supply 1 is approximated to the peak value, the input current may flow. When the power supply voltage is not substantially equal to the peak value, no input current will flow.
As a consequence, the waveform of the input current becomes a pulsatory waveform, and the peak value thereof is also increased.
In the conventional inverter apparatus with the above-described circuit arrangement, since the inverter apparatus is of the capacitor input type, the waveform of the input current becomes the pulsatory peak shape. There are problems that the power factor is lowered and the harmonic disturbance will occur.